Fire resistance of austenitic stainless steels: Polarit 725 (EN 1.4301) and Polarit 761 (EN 1.4571)

The main purpose of the study was to determine the behaviour of austenitic stainless steel at elevated temperatures. The stress-strain relationship of a material at elevated temperatures is required to determine the load-bearing capacity of structures under fire conditions. The study is limited to austenitic stainless steels, which are the most commonly used group for structural applications. Stress-strain relationships at elevated temperatures were determined with steady-state tensile tests at elevated temperatures for two austenitic stainless steels Polarit 725 (conforming to material number EN 1.4301 and AISI 304) and Polarit 761 (conforming to material number EN 1.4571 and AISI 316 Ti). The material properties were determined for both virgin sheet and cold-worked material; steady-state tensile tests were performed up to 900 °C and 950 °C respectively. At temperatures above 500 - 600 °C the yield strength (stress at a proof strain of 0.2%) of austenitic stainless steels Polarit 725 and Polarit 761 does not decrease as strongly as that of carbon steels determined according to Eurocode 3, Part 1.2 (ENV 1993-1-2 1995). As much as 50% of the yield strength of Polarit 761 remains at 800 °C. It should noted, however, that mechanical properties determined with steady-state tests are optimistic compared with transient-state tensile test results when only small strains are involved. On the other hand, the yield strength values of carbon steel determined according to Eurocode 3, Part 1.2 correspond to a total strain of 2%. The modulus of elasticity of austenitic stainless steels decreases more slowly than that of carbon steels. Increased strength due to the cold-forming process remains constant up to 600 °C, after which the strength begins to decrease and the influence of cold-forming totally disappears at 900 °C. The elongation to fracture of cold-formed material is much lower than that of the virgin sheet. The load-bearing capacities of certain stainless steel structures were calculated. The thermal material properties used in the calculations were based on the literature. The results of this study are promising concerning the application possibilities of austenitic stainless steels when fire resistance is required.